In flip-chip mounting, generally, protruded electrodes such as solder bumps are formed on electrodes of semiconductor elements such as LSIs, and the semiconductor elements with the protruded electrodes formed thereon are mounted face-down on mounting boards. Specifically, the protruded electrodes on the semiconductor elements are pressed against electrode terminals on the mounting boards while being heated. A typical method for forming solder bumps on electrodes of semiconductor elements includes forming solder layers on the electrodes by screen printing, dispensing, or electrolytic plating, and then heating the solder layers to at least the melting point of solder in a reflow furnace.
In recent years, electrode terminals of semiconductor elements have been arranged with narrower pitch and smaller area in order to facilitate increase in the density and number of electrode terminals of semiconductor elements. The reduction in the pitch of electrode terminals of semiconductor elements may cause short-circuits between the electrode terminals in the case where the electrode terminals are arranged in a line or two lines in a zigzag pattern along the outer peripheries of the semiconductor elements as in the related art. Further, the reduction in the pitch of electrode terminals of semiconductor elements may warp mounting boards due to differences in thermal expansion coefficient between the semiconductor elements and the mounting boards, thereby causing connection failure. Thus, an area array has been adopted in which the electrode terminals of semiconductor elements are arranged in a matrix pattern, thereby increasing the pitch between the electrode terminals.
In recent years, however, electrode terminals have been further arranged with narrower pitch even in an area array, so that solder joints have been arranged with narrower pitch. Moreover, gaps have been narrowed between semiconductor elements and circuit board terminals. For these reasons, in the pressing and heating step of flip-chip mounting, solder bridge defects may be caused. Solder bridge defects may be caused with molten and transformed solder bumps connected to each other by the surface tension of solder.
Thus, there has been proposed a semiconductor device in which protruded electrodes made of gold or copper are covered by an insulating film containing metal particles (for example, see Patent Literature 1). In the semiconductor device, the insulating film and the protruded electrodes are not molten during flip-chip mounting. A force is generated in a compressing direction when sealing resin between a semiconductor element and a circuit board is cured and contracted. The force causes the metal particles contained in the insulating film to contact the protruded electrodes and the circuit board terminals, thereby electrically connecting the protruded electrodes of the semiconductor element and the circuit board terminals. Thus, the semiconductor device can prevent the occurrence of bridge even when electrode terminals are arranged with narrower pitch.
However, in the connection configuration in which electrical conduction is obtained only by the contact of the metal particles with the protruded electrodes and the circuit board terminals without diffusion bonding, as a matter of course, as the electrode area of the semiconductor element decreases, the number of conductive particles between the protruded electrodes and the circuit board terminals decreases. Thus, resistive connection increases, thereby increasing the transmission loss of signals. Particularly in recent years, since electrode terminals of semiconductor elements have been stringently required to be arranged with narrower pitch, the electrode terminals of the semiconductor elements have been further reduced in area, thereby adding to the foregoing problem.
Thus, there has been adopted a protruded electrode having a two-layer structure which includes a lower-layer metal and an upper-layer metal. The lower-layer metal is a high-melting-point metal, and the upper-layer metal is formed on the lower-layer metal by solder (for example, see Patent Literature 2). In the protruded electrode having a two-layer structure, the amount of solder can be made smaller than that in a protruded electrode formed of only a solder layer, thereby reducing the amount of solder protruded in a planar direction during flip-chip mounting. This can prevent the occurrence of solder bridge. Further, the protruded electrode having a two-layer structure causes solder and a circuit board terminal to be diffusion-bonded, thereby reducing resistive connection. Thus, the transmission loss of signals does not increase.